The objective of the proposed research is to find a new index of vigor of cardiac contraction. The index must be independent of end-diastolic ventricular volume and afterloaded systolic pressure because these change all the time. At the same time, the index must be sensitive to the vigor of the muscle. In dog experiments, we have shown that the time-varying ratio, E(t), of intantaneous intraventricular pressure, P(t), to instantaneous intraventricular volume, V(t), serves as a good index of contractility. E(t), defined as P(t)/(V(t)-Vd), was independent of end-diastolic ventricular volume and systolic pressure when the contractile state was maintained constant, whereas it was very sensitive to changes in the inotropic state of the ventricle. Vd in the equation represents that part of the ventricular volume which does not participate in active pressure generation. Since the use of pressure volume ratio as a new index of ventricular contractility is well indicated, we propose to study whether the time-varying pressure-diameter ratio of the dog's ventricle ED(t) has similar characteristics as the pressure-volume ratio, EV(t). Ventricular internal diameter is to be measured by ultrasonic devices. First, we studied both ED(t) and EV(t) simultaneously in excised dog's hearts to analyze the correlation between the two ratios. We will now confirm in conscious dogs with denervated heart that ED(t) is independent of the preload and afterload as was EV(t). Finally, we plan to study ED(t) in conscious dogs with intact cardiac nerves to quantitate neural and humoral control of ventricular contractility. In these experiments other indexes of ventricular contractility (e.g., dP/dt and isometric wall force) will be compared against ED(t) to evaluate its use and limitation.